1. Field of Art
The present invention relates generally to a process for the treatment of waste fluids, and more particularly to a process for treating the waste discharged from paper making factories in an enormous quantity in a manner such that the organic substances contained in the waste are fermented efficiently to generate methane which may be used as an economical resource.
Kraft pulps are predominantly prepared and used in the paper making industry. Particularly in Japan, the share of the kraft pulps reaches more than 70% of the total yield of pulps. In order to prepare 1 ton of kraft pulp, about 2 tons of wooden material and about 0.5 tons of an alkali are put into a pulping kettle or pulper with a large volume of water. In this pulp making step, materials other than cellulose are dissolved in water to form the so-called black liquor or cooked liquor, the main ingredient of the black liquor being lignin. The black liquor is concentrated by evaporating water in a boiler so that the alkali and alkali compounds are recovered so as not to cause pollution problem due to contamination thereby. The liquid extracted from the black liquor at the evaporation step is commonly referred to as evaporator condensate. The amount of the evaporator condensate discharged from the paper or pulp making factory is so large that a factory having a capacity of producing pulp of 1000 tons/day, for example, discharges the evaporator condensate in an amount of about 6000 to 7000 tons/day. Under these circumstances, it becomes an important problem to treat the waste in a manner such that the profitable materials contained therein is converted into useful substances of some form and water can be reused in the pulp making step.
2. Statements of Related Art
Up to date, the waste from the pulp making step containing several thousand ppm of organic materials has been processed through the activated sludge process together with other wastes. However, the concentration of organic materials contained therein is generally too high to be conveniently processed through the activated sludge process; while the concentration of organic materials adapted for successful processing by the activated sludge process is not less than about 1000 ppm, that in the waste from the pulp making system ranges, in general, from 3000 to 8000 ppm. It has been, therefore, necessitated either to dilute the waste to lower the concentration of organic materials contained therein or the residence time of such waste in the processing tank or vessel must be prolonged, leading to enormous increase in cost.
On the other hand, if the waste can be used as a raw material for a methane fermentation system, the concentration of organic materials in a raw material required for such fermentation being normally about 20,000 ppm or more, the waste may be used as an energy source in addition to the advantageous result that the loading on the conventional activated sludge process can be reduced, whereby favourite economical interests including saving of natural resources and saving of energy are attainable. With such the object in view, it has been eagerly investigated to develop a methane fermentation process using a bio-reactor in recent years. However, there has not yet been developed a satisfactory methane fermentation process for the treatment of the waste from the pulp making system, to the best of our knowledge.
Although it is not intended to be bound to any theory or hypothesis, it is considered that the content of organic materials contained in such a waste, 3000 to 8000 ppm as aforementioned, is lower than that suited for methane fermentation, and that a large amount of sulfur compounds which are recognized as inhibitors for anaerobic fermentation is contained therein to inhibit or hinder continuous and stable methane fermentation.